1. Field of the Invention
The invention relates to a method of producing a fuel cell. The invention more particularly relates to a method of producing a fuel cell that is provided with an electrode layer that contains carbon nanotubes (CNTs).
2. Description of Related Art
For example, Japanese Patent Application Publication No. 2009-110768 (JP-A-2009-110768) discloses a fuel cell production method that uses, for the polymer electrolyte present in the electrode layer, a polymer electrolyte that has a higher glass-transition temperature than the polymer electrolyte that forms the electrolyte membrane, and that carries out bonding between the electrode layer and electrolyte membrane at a temperature between the glass-transition temperatures of these polymer electrolytes. Specifically, an ink is first prepared by mixing finely divided, catalyst-loaded carbon particles with a solution of the polymer electrolyte (ionomer) and this ink is dried. An electrode layer can thereby be fabricated in which voids are formed between the finely divided carbon particles. The fabricated electrode layer and an electrolyte membrane are bonded at a temperature that is lower than the glass-transition temperature of the ionomer, and higher than or equal to the glass-transition temperature of the polymer electrolyte used in the electrolyte membrane. The execution of bonding using this temperature condition can inhibit softening of the ionomer during bonding. As a consequence, the electrode layer and electrolyte membrane can be adhered to each other while maintaining the voids between the finely divided particles that were formed during fabrication of the electrode layer.
Fuel cells that use CNT in the electrode layer in place of the aforementioned finely divided carbon particles are available. Moreover, fuel cells are also available in which this CNT is aligned vertically to the plane of the electrolyte membrane. An electrode layer that uses such vertically aligned CNT has a structure in which voids are formed between adjacent CNTs along the tube length direction of the CNTs. A high porosity can be imparted to the electrode layer as a result. However, a high void ratio also means a low carbon density. A drawback of an electrode layer that uses vertically aligned CNT is therefore facile detachment when bonded with the electrolyte membrane.
Accordingly, when vertically aligned CNT is used in the electrode layer, a method has been adopted in which bonding is carried out by applying higher pressure than in the use of finely divided carbon particles, while also softening the ionomer and/or the polymer electrolyte of the electrolyte membrane. Strong adhesion between the electrode layer and electrolyte membrane can be obtained using this method. However, when such a bonding method is employed, the vertically aligned CNT may tilt with respect to the direction vertical to the plane, i.e., collapse may occur. This has resulted in the appearance of the defective assembly.